This invention relates to digital imaging systems, and more particularly to a method for providing a wide dynamic range with acceptable signal-to-noise ratio in digital imaging systems and digital imaging systems operable for use with the method.
Non-destructive testing may utilize a number of different technologies, including x-ray, ultrasound, and infra-red imaging. About 25% of all non-destructive testing examinations are currently performed using x-rays. Digital x-ray imaging devices, in particular, are useful for non-destructive imaging. The gamut of applications used in x-ray non-destructive testing covers a large energy spectrum from low KeV to MeV applications, for example from about 40 KeV to about 10 MeV, covering items such as printed circuit boards, wax casting, metal casting, turbine blades, rocket cones, and the like, without limitation. Thus, systems capable of providing large-area, high-resolution images over a wide dynamic range of energy levels are desirable for satisfactory performance in multiple applications.
The features of the objects examined during digital imaging are generally xe2x80x9cphoton limited,xe2x80x9d meaning that high radiation exposures are used so that low-contrast details can be differentiated. These applications range from low signal-to-noise ratio requirements to very high signal-to-noise ratio requirements. Typically, high image contrast is desired, which requires a high signal-to-noise ratio. The desire for high signal-to-noise ratio limits the number of applications for which a given system can be optimized. Thus, to optimally satisfy all the x-ray non-destructive testing applications, a range of imaging systems may be needed, as any one system currently cannot meet the signal-to-noise ratio requirements for the entire energy range.
Screen-film is the optimal standard of analog imaging against which digital systems are typically compared. Many digital x-ray systems currently in use and under development, some based upon charge-coupled device (CCD) technology, others based upon photostimulated storage phosphor technology, and still others based upon thin-film-transistor (TFT) technology, to name a few. Some of these digital systems are comparable to film in performance, but not over the entire range of applications.
It is desirable, therefore, to provide a method for controlling the dynamic range and signal-to-noise ratio of a digital x-ray system so that a single device may be used over a wide range of applications and energy levels. In particular, it is desirable to provide such method with respect to TFT-based digital x-ray systems.
The invention comprises a digital x-ray imaging device and method. The imaging device comprises a top electrode layer; a dielectric layer; a sensor layer comprising a photoconductive layer and a plurality of pixels, each pixel comprising a charge-collecting electrode; a thin film transistor (TFT) readout matrix connected to the charge-collecting electrodes; and a variable power supply adapted to provide a range of voltages between the top electrode layer and the TFT readout matrix. The variable power supply may comprise a programmable power supply, and may have approximately a 2:1 turndown ratio, such as providing a range of voltages between about 1.5 kV and about 3.0 kV.
For a digital x-ray imaging device such as that described above, the invention comprises a method for providing a broad dynamic range for the device. The method comprises varying the voltage between the top electrode layer and the TFT readout matrix to provide an acceptable signal-to-noise ratio over a greater range of exposures than provided at a single voltage. The method may be used for non-destructive testing of one or more objects, such as but not limited to printed circuit boards, wax castings, metal castings, turbine blades, and a rocket cones. The method may include providing a signal-to-noise ratio of at least about 50.